Modern, wind-driven electricity generators were born in the late 1970s. Until the early 1970s, wind energy filled a small niche market supplying mechanical power for grinding grain and pumping water, as well as electricity for rural battery charging. With the exception of battery chargers and rare experiments with larger electricity-producing machines, the windmills of 1850 and even 1950 differed very little from the primitive devices from which they were derived. As of July 2008, wind energy provides approximately 1% of total U.S. electricity generation. Smaller designs with as many as 30 blades are common for irrigation in farming.
Most modern wind turbines typically have 3-bladed rotors 10 with diameters of 10-80 meters mounted atop 60-80 meter towers 112. The average turbine installed in the United States in 2006 can produce approximately 1.6 megawatts of electrical power. Turbine power output is controlled by rotating the blades around their long axis to change the angle of attack (pitch) with respect to the relative wind as the blades spin around the rotor hub. The turbine is pointed into the wind by rotating the nacelle around the tower (yaw). Turbines are typically installed in arrays (farms) of 30-1150 machines. A pitch controller (for blade pitch) regulates the power output and rotor speed to prevent overloading the structural components—during unusually strong wind conditions. Generally, a turbine will start producing power in winds of about 5.36 meters/second and reach maximum power output at about 12.52-13.41 meters/second (28-30 miles per hour). The turbine will pitch or feather the blades to stop power production and rotation at about 22.35 meters/second (50 miles per hour).
Efforts have been made to provide mechanisms to transition the blades from the open deployed position, in which the blades are generally parallel to the mast and radially about the rotating hub, to the closed retracted position (folded back) in which the blades are generally perpendicular to the mast in a tight cluster. This allows for reducing the surface area in high wind environments, as well as for storage and ease of transport. U.S. patent application Ser. No. 12/461,716 and Ser. No. 12/461,575 incorporated herein both disclose designs for opening and closing the blades.